Device and method for preparing large-sized high-quality aluminium alloy ingot

ABSTRACT

Provided is a device for preparing a large-sized high-quality aluminium alloy ingot, which is mainly composed of a uniform cooler, a hot top, an oil-gas lubrication mold, an induction coil and a dummy ingot, wherein the hot top is arranged above the oil-gas lubrication mold, the induction coil is arranged outside the oil-gas lubrication mold, the uniform cooler is arranged inside the oil-gas lubrication mold, and the dummy ingot is arranged below the oil-gas lubrication mold. Further provided is a method for preparing a large-sized high-quality aluminium alloy ingot. The device combines a partitioned gas supply mold with the uniform cooler and an electromagnetic stirrer, and the effective coupling of the three achieves forced and uniform solidification forming of a melt under gas pressure contact conditions, such that a stable and continuous gas film is formed between the melt and the mold. The ingot has a smooth surface, and a fine and uniform internal structure.

TECHNICAL FIELD

The present invention belongs to the field of metal material processing,and more particularly, to a device and method for preparing alarge-sized high-quality aluminium alloy ingot.

BACKGROUND OF THE INVENTION

As improved operational performances have been achieved for large-sizedwhole-set equipment in the manufacturing industry, it has become aninevitable trend that large-sized integrated structures will be usedmore and more widely in such fields as aerospace, rail transportationand shipbuilding. For example, large-sized integrated aluminiummaterials and high-performance thick plates are widely employed todevelop large-sized transport aircrafts, high-performance combataircrafts and high-speed trains. Moreover, as these materials areprepared by large-sized high-quality aluminium alloy ingots, thepreparation of the latter is of great significance to improving theequipment capability of the manufacturing industry.

The semi-continuous casting process is the primary method for producingaluminium alloy ingots. However, common semi-continuous castingtechniques are subjected to certain limitations, and therefore, duringthe preparation of large-sized aluminium alloy ingots, they tend to besolidified from outside to inside because of limited cooling manners.This will certainly lead to a deep liquid cave for molten metal.Furthermore, as ingots are large in dimensions, the uniformity oftemperature fields would be difficult to control during solidification,which, therefore, would lead to a non-uniform solidified shell. In suchcases, such issues as wrinkles, segregation humps and even breakoutwould be prone to occur at thinner areas of the solidified shell. Inaddition, as ingots are large, and cooling effects are limited, thespeed of continuous casting would certainly be reduced. As such, thenuclei formed in the melt would be few and non-uniform, and grains wouldbe unusually coarse. Therefore, large-sized aluminium alloy ingotsprepared by the traditional semi-continuous casting method are featuredby poor surface quality, coarse and non-uniform internal structures,severe component segregation and low ingot yield, which need to besubjected to surface or face milling prior to deformation processing,thereby leading to high costs and serious waste of materials.

To solve such problems, researchers have carried out a lot ofinvestigations, hoping to prepare non-segregated aluminium alloy ingotsfeatured by fine and uniform internal structures as well as good surfacequality.

Chinese Patent CN104550798A proposes an aluminium alloy semi-continuouscast electromagnetic stirring device and method. In this method, bycombining direct current with permanent magnets, the flow modes and flowintensities of different melts in the mold region are designed accordingto sizes, shapes and material components of aluminum ingots, andsegregation behaviors of alloying elements and growth patterns ofdendrits are controlled to realize structure homogenization andrefinement. However, as this invention employs a single cooling mannerin which solidification is achieved sequentially from outside to insidesimply by means of a mold, the problem of temperature non-uniformitythat occurs during solidification of the aluminium alloy melt stillremains unsolved. Particularly, for the preparation of large-sizedingots, the stirring effects of the above-mentioned method on the meltare limited. Consequently, temperature gradients are large within themelt, and the liquid cave is very deep. In such cases, the speed ofcasting is extremely slow, and thus, the improvement effects of thismethod on the internal quality of ingots are limited.

With respect to improvements on the surface quality of ingots, the gasfilm casting method, which is represented by the Airslip technique fromAmerica and the AirsolVeil technique from German, works as follows: alayer of gas film is formed between a mold and an ingot's solidifiedshell so as to reduce the contact pressure between the solidified shelland the inner wall of the mold during solidification, thereby achievingsolidification forming of a melt under gas contact conditions. Based onthis, Chinese Patent CN100418667C makes improvements on the oil-gaslubrication mold, and proposes a mold having integral design of oil, airand water structure. In this patent, primary cooling is omitted becauseof the heat insulation effects of the oil-gas film, and secondary watercooling effects are enhanced through two rows of water spraying holes,thereby improving the surface quality of an ingot. However, the priorgas film casting method can hardly be used to prepare a high-qualitylarge-sized ingot. If the large-sized ingot is cooled simply by themold, the speed of casting would be slow, and the initial solidifiedshell formed after the melt is cooled in the mold would be very thin andnon-uniform. Moreover, as for the oil film casting technique, oil gascan easily penetrate through the initial solidified shell, thus leadingto such problems as run-out and breakout. As such, the productionprocess is hard to control, and generally, the diameter of the ingotcan't exceed 300 mm (12 inches). Furthermore, the gas film castingtechnique can't solve such problems as tiny and uniform solidifiedstructures as well as component segregation.

SUMMARY OF THE INVENTION

The prior gas film casting method can't be used to prepare large-sizedaluminium alloy ingots, as it has the disadvantage that the ingotsprepared thereby are featured by poor surface quality, and coarse andnon-uniform internal structures. In view of the drawbacks of the priorsemi-continuous casting method with respect to the preparation oflarge-sized aluminium alloy ingots, the present invention provides a newdevice and method for preparing a large-sized high-quality aluminiumalloy ingot. In this invention, a melt is subjected to a combination ofintra-mold intermediate uniform cooling and extra-mold electromagneticstirring during the gas film casting operation. Therefore, when alarge-sized high-quality aluminium alloy ingot is prepared, the problemsof surface quality and internal quality that traditionally occur can besolved simultaneously.

The main design idea of this invention is as follows: given that theprior gas film casting method can hardly be used to prepare large-sized(the diameter is greater than 300 mm) aluminium alloy ingots, the moldin the present invention is designed to be of a partitioned gas supplystructure, which reduces the difference in gas supply pressure on thegraphite ring, and achieves stable control of gas pressure, therebyensuring that a stable and continuous gas film can be formed between themelt and the mold. Moreover, the alloy melt in the present invention issubjected to a combination of intra-mold uniform cooling and extra-moldelectromagnetic stirring during the semi-continuous casting operation,thereby increasing the cooling dimensions of the ingot duringsolidification, strengthening the three-dimensional convection of themelt during solidification, and improving the uniformity of temperatureand component fields for the bulky alloy melt. Furthermore, in additionto ensuring the internal quality of the ingot, the present invention isalso intended to improve the uniformity of initial solidification,increase the thickness of the initially solidified shell, and preventthe initially solidified shell and the oil-gas film from fracturing.Besides, it also aims to reduce the contact pressure between theinitially solidified shell and the inner wall of the mold so as toachieve solidification forming of a melt under gas pressure contactconditions, thereby preparing a large-sized aluminium alloy ingot withexcellent surface quality and internal quality.

A new device for preparing a large-sized high-quality aluminium alloyingot is provided, which is mainly composed of a uniform cooler, a hottop, an oil-gas lubrication mold, an induction coil and a dummy ingot,wherein the hot top is arranged above the oil-gas lubrication mold, theinduction coil is arranged outside the oil-gas lubrication mold, theuniform cooler is arranged inside the oil-gas lubrication mold, and thedummy ingot is arranged below the oil-gas lubrication mold.

The oil-gas lubrication mold comprises a mold body and a graphite ringmounted above the mold body. The graphite ring is provided with a gasgroove and an oil groove on its outer wall, wherein the gas groove isdivided into 3-20 sections, and each section of the gas groove has alength of 100-500 mm, and is provided independently with an air intakepassage for independent gas supply and control; the oil groove isseparated from the gas groove, and is arranged on an upper portionthereof. The graphite ring is prepared by porous graphite, and gas andlubricating oil seep out of the mold through the graphite ring. Theoil-gas lubrication mold is designed to be of a partitioned gas supplystructure, which may reduce the difference between the gas amount andgas pressure within the gas groove of a single gas supply graphite ring,thereby achieving the object of stably controlling gas pressure.

The oil-gas lubrication mold is provided with two rows of water sprayingholes. The mold body is provided with upper and lower rows of waterspraying holes, wherein the upper row of water spraying holes forms anangle of 15-30 degrees with respect to a wall of the mold, and has adiameter of 1-5 mm; while the lower row of water spraying holes forms anangle of 0-25 degrees with respect to the wall of the mold (this angleis greater than 0 degree, such that cooling water can be ensured to besprayed to an ingot without being splashed back), and has a diameter of2-8 mm. Water volumes of the two rows of water spraying holes may beindependently controlled, and the diameter of the upper row of waterspraying holes needs to be smaller than or equal to that of the lowerrow of water spraying holes.

The induction coil is arranged outside the mold. A magnet yoke (ironcore) is of a telescopic design, which is variable in length, andretractable in a range of 0-100 mm. The electromagnetic induction coilgenerates an electromagnetic field which is guided into a melt insidethe mold via the magnet yoke. The electromagnetic coil may generate arotating electromagnetic field, a traveling wave electromagnetic fieldor a compound electromagnetic field.

The upper portion of the uniform cooler is a heat insulation end, andthe lower portion is a cooling end, wherein the heat insulation end isprovided with a stirring blade. During the semi-continuous castingoperation, the uniform cooler passes through the hot top and stretchesto a height where the mold is located, and its bottom portion is flushwith the mold. The uniform cooler may be arranged to be one or more innumber, and rotate at a rotational speed of 0-300 r/min.

The heat insulation end is of a cylindrical shape, and has an outerdiameter of 100-800 mm. It is made of high temperature-resistant heatinsulation ceramic material, which has heat insulation effects, therebypreventing the melt in the hot top from being cooled; the cooling end ismade of thermally conductive material (e.g., graphite, copper,molybdenum, titanium and composite materials thereof), and has coolingeffects. The cooling end of the uniform cooler is of a spiral shape, andthus, the rotation of the uniform cooler will force a melt to flowdownward. The stirring blade is made of high temperature-resistantmaterial (e.g., copper, molybdenum, titanium, ceramic and compositematerials thereof), which is arranged to be 0-8 in number, and has awidth of 10-100 mm and a thickness of 2-8 mm. It rotates along with theuniform cooler to force a melt to flow downward, such that the melt issupplemented downward into the liquid cave in a constant manner, therebyexhibiting dynamic and continuous uniform cooling effects. A circulatingcooling medium is introduced into the uniform cooler, and reaches thecooling end through which it exchanges heat with the melt; this coolingmedium may be air, nitrogen, water, oil and various other fluids, andhas a flow of 0-2000 L/min.

Based on the above device, the present invention provides a method forpreparing a large-sized high-quality aluminium alloy ingot. In themethod, a melt that has been refined and stabilized to be 80-100 degreesCelsius higher than the liquidus temperature is poured into the hot topduring the semi-continuous casting operation; air and lubricating oilare introduced into the gas groove and the oil groove arranged on theouter wall of the graphite ring; flows of the upper and lower waterspraying holes are controlled; an alloy melt reaches the upper portionof the dummy ingot through the hot top and the mold, and the liquidsurface of the melt is elevated to a desired height; after thecontinuous casting operation is initiated, the dummy ingot descendsslowly, and the flow of cooling water is increased slowly; after thecasting process is stabilized, uniform cooling and electromagneticstirring are applied to obtain a large-sized high-quality aluminiumalloy ingot in the end.

During the semi-continuous casting operation, the flow of air in the gasgroove is 500-5000 mL/min, while the oil groove supplies oil in a pulsedmanner, and has an oil supplying capacity of 60-100/s; the flow of theupper row of water spraying holes is 1-50 L/min, while that of the lowerrow of water spraying holes is 20-100 L/min; the speed of casting is20-100 mm/min. The cooling intensity of the uniform cooling is 500-5000W/(m²·k), and the shearing rate of the electromagnetic stirring is10-2000 s⁻¹.

The innovation and technical progress of the present invention aremainly manifested by the following aspects:

1. During the semi-continuous casting operation of the presentinvention, the design of a partitioned gas supply structure for theoil-gas lubrication mold is artfully combined with the application ofintra-mold intermediate uniform cooling and extra-mold electromagneticstirring, and the intercoupling of the cooling effects of the uniformcooler, the structure and rotational speed of the stirring blade and theshearing strength of the electromagnetic stirring may be controlled toachieve forced uniform cooling and three-dimensional convection for themelt as a whole. As such, the uniformity of temperature and componentfields is significantly improved while the cooling intensity isincreased. This not only fundamentally solves such problems as coarseand non-uniform structures, macrosegregation and cracking present in thelarge-sized aluminium alloy ingot prepared by the common semi-continuouscasting method, but also greatly improves the uniformity of theinitially solidified shell, and increases the thickness thereof. As thecontact pressure between the initially solidified shell and the innerwall of the mold is effectively reduced, solidification forming of amelt may be achieved under gas pressure contact conditions, thussignificantly improving the surface quality of the ingot.

2. The oil-gas lubrication purifier for large-sized aluminium alloy isdesigned to be of a partitioned gas supply structure, which may reducethe difference between the gas amount and gas pressure within the gasgroove of a single gas supply graphite ring, thereby stably controllingthe gas pressure; oil is supplied in a pulsed manner, such that a stableand continuous gas film can be formed between the melt and the mold soas to reach the effect of stable lubrication; this manner solves thetechnical problem that the gas film casting method can't be used toprepare the large-sized aluminium alloy ingot (whose diameter is greaterthan 300 mm), and the prepared ingot is smooth in surface.

3. The large-sized ingot prepared by the present invention is featuredby fine grains, uniform components and smooth surface. The speed ofcasting is rapid, which significantly reduces the costs resulted fromsubsequent homogenization and processing operations, thereby improvingthe production efficiency and the pass percent. The whole set of methodis simple and feasible, and has good implementation effects, which maybe utilized to achieve industrial production.

In the present invention, the partitioned gas supply mold is artfullycombined with the uniform cooler and the electromagnetic stirrer,wherein the design of the partitioned gas supply mold can achieve stablecontrol of gas pressure, the uniform cooler increases the coolingdimensions of the ingot during solidification, and the electromagneticstirrer strengthens the three-dimensional convection of the melt duringsolidification. As such, the uniformity of temperature and componentfields of the bulky alloy melt is improved. The effective coupling ofthe three units can achieve forced and uniform solidification forming ofa melt under gas pressure contact conditions, such that a stable andcontinuous gas film can be formed between the melt and the mold. Theprepared ingot has not only a smooth surface, but also a fine anduniform internal structure. The large-sized high-quality aluminium alloyingot prepared by the present invention is featured by a high productionefficiency, and can readily be combined with large-scale industrialproduction. Therefore, the device and method of the present inventionhave a broad industrial application prospect in such manufacturingfields as aerospace, rail transportation and ships.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the structure of a device ofthe present invention for preparing a high-quality large-sized aluminiumalloy ingot.

FIG. 2 is a schematic diagram illustrating the partitioning of agraphite ring.

FIG. 3 is an enlarged diagram illustrating a partial area E of a mold ofFIG. 1.

FIG. 4 is a schematic diagram illustrating a uniform cooler.

FIGS. 5a and 5b are pictures illustrating the surface appearances of7075 aluminium alloy ingots (Φ=582 mm) prepared respectively by thecommon semi-continuous casting method and the present invention.

FIGS. 6a and 6b illustrate the microstructures of 7075 aluminium alloyingots (Φ=582 mm) prepared respectively by the common semi-continuouscasting method and the present invention.

Main reference numerals are illustrated as follows:

 1 - uniform cooler;  2 - hot top;  3 - melt;  4 - oil-gas lubricationmold;  5 - magnet yoke;  6 - coil;  7 - graphite ring;  8 - waterspraying holes;  9 - ingot; 10 - dummy ingot; 11 - oil groove; 12 - gasgroove; 13 - upper row of water spraying 14 - lower row of waterspraying holes; holes; 15 - heat insulation end; 16 - stirring blade;17 - cooling end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be implemented based on the followingembodiments, but not limited thereto. These embodiments are merely forthe purpose of illustrating the implementation process of the presentinvention, and not intended to limit the scope of the present inventionin any way. In the following embodiments, various processes and methodsthat have not been described in detail are conventional methods known inthe art.

As shown in FIG. 1, the device of the present invention comprises auniform cooler 1, a hot top 2, a melt 3, an oil-gas lubrication mold 4,a magnet yoke 5, a coil 6, a graphite ring 7, water spraying holes 8, aningot 9, a dummy ingot 10, etc. The hot top 2 is arranged above theoil-gas lubrication mold 4, the coil 6 and the magnet yoke 5 arearranged outside the oil-gas lubrication mold 4, the uniform cooler 1 isarranged inside the oil-gas lubrication mold 4, and the dummy ingot 10is arranged below the oil-gas lubrication mold 4.

The oil-gas lubrication mold 4 comprises a mold body and a graphite ring7 mounted above the mold body. The graphite ring 7 is provided with agas groove 12 and an oil groove 11 on its outer wall, wherein the gasgroove 12 is of a sectional design. As shown in FIG. 2, the gas groove12 is divided into 3-20 sections, and each section of the gas groove 12has a length of 100-500 mm, and is provided independently with an airintake passage for independent gas supply and control; the oil groove 11is separated from the gas groove 12, and is arranged on an upper portionthereof. The graphite ring 7 is prepared by porous graphite, and gas andlubricating oil seep out of the mold through the graphite ring 7.

The oil-gas lubrication mold 4 is provided with two rows of waterspraying holes, as shown in FIG. 3; water volumes of the two rows ofwater spraying holes are independently controlled; the upper row ofwater spraying holes 13 forms an angle of 15-30 degrees with respect toan inner wall of the oil-gas lubrication mold 4, and has a diameter of1-5 mm; while the lower row of water spraying holes 14 forms an angle of0-25 degrees (greater than 0 degree) with respect to a wall of theoil-gas lubrication mold 4, and has a diameter of 2-8 mm; the diameterof the upper row of water spraying holes needs to be smaller than orequal to that of the lower row of water spraying holes.

The magnet yoke 5 is of a telescopic design. The magnet yoke 5 isvariable in length, and is retractable in a range of 0-100 mm. Theelectromagnetic induction coil 6 generates an electromagnetic fieldwhich is guided into a melt inside the mold via the magnet yoke 5. Theelectromagnetic coil 6 may generate a rotating electromagnetic field, atraveling wave electromagnetic field or a compound electromagneticfield.

During the semi-continuous casting operation, the uniform cooler 1passes through the hot top 2 and stretches to a height where the oil-gaslubrication mold 4 is located. The uniform cooler 1 may be arranged tobe one or more in number, and rotate at a rotational speed of 0-300r/min. As shown in FIG. 4, the uniform cooler 1 is composed of an upperheat insulation end 15 and a lower cooling end 17, and the heatinsulation end 15 is provided with a stirring blade 16; the upper heatinsulation end 15 is of a cylindrical shape, which has an outer diameterof 100-800 mm, and is made of high temperature-resistant heat insulationmaterial; the cooling end 17 is made of thermally conductive material,such as graphite, copper, molybdenum, titanium and composite materialsthereof; the cooling end 17 of the uniform cooler 1 is of a spiralshape, and thus, the rotation of the uniform cooler 1 will force a meltto flow downward; the stirring blade is arranged to be 0-8 in number,and has a blade width of 10-100 mm and a thickness of 2-8 mm; thestirring blade 16 is made of high temperature-resistant material, suchas copper, molybdenum, titanium, ceramic and composite materialsthereof, and rotates along with the uniform cooler 1; during operation,it will drive the melt to converge towards the cooling end 17 of theuniform cooler 1; a circulating cooling medium is introduced into theuniform cooler 1, and reaches the cooling end 17 through which itexchanges heat with the melt, wherein the cooling medium may be air,nitrogen, water, oil and various other fluids, and has a flow of 0-2000L/min. To achieve the continuous and dynamic uniform supercooling of amelt, the melt is made to pass through the bottom portion of the uniformcooler for cooling, and then continues to flow downward into the mushyzone of a liquid cave, thereby achieving the continuous and dynamicuniform cooling of the melt as well as forced feeding, and preparing alarge-sized fine-grained homogeneous ingot.

The application method is as follows: during the semi-continuous castingoperation, the overall device is preheated to 80-200 degrees Celsius,and a melt that has been refined and stabilized to be 80-100 degreesCelsius higher than the liquidus temperature is poured into this device.During the continuous casting operation: air and lubricating oil areintroduced into the gas groove 12 and the oil groove 11 arranged on theouter wall of the graphite ring 7, wherein the flow of air is 500-5000mL/min, while oil is supplied in a pulsed manner, with the oil supplyingcapacity being 60-100/s; the electromagnetic coil 6 is initiated, andthe current is 10-200 A; the flow of the upper row of water sprayingholes 13 is controlled to be 1-50 L/min, while that of the lower row ofwater spraying holes 14 is controlled to be 20-100 L/min; the speed ofcasting is 20-100 mm/min.

An alloy melt reaches the upper portion of the dummy ingot 10 throughthe hot top 2 and the mold 4, and the liquid surface of the melt iselevated to a desired height; after the continuous casting operation isinitiated, the dummy ingot 10 descends slowly, and the flow of coolingwater is increased slowly; after the casting process is stabilized,uniform cooling and electromagnetic stirring are applied until thecasting process is completed, wherein the cooling intensity of theuniform cooling is 500-5000 W/(m²·k), and the shearing rate of theelectromagnetic stirring is 10-2000 s⁻¹.

The 7075 aluminium alloy rounded ingot (Φ=582 mm) prepared by thepresent invention is required to have a smooth surface, and a fine anduniform internal structure. The specific implementation is as follows:

The structural schematic diagram of the device is as shown in FIG. 1.The oil-gas lubrication mold 4 employs a partitioned gas supply system,and the graphite ring 7 is provided externally with a gas groove 12 andan oil groove 11, wherein the gas groove 12 is divided into 4 sections,and each section of the gas groove 12 has a length of 456 mm, and isprovided independently with an air intake passage for independent gassupply and control; the oil groove 11 is separated from the gas groove12, and is arranged on an upper portion thereof, and the graphite ring 7is prepared by porous graphite. The upper row of water spraying holes 13forms an angle of 25 degrees with respect to a wall of the mold, and hasa diameter of 2 mm; while the lower row of water spraying holes 14 formsan angle of 10 degrees with respect to the wall of the mold, and has adiameter of 5 mm. Water volumes of the two rows of water spraying holesmay be independently controlled.

The uniform cooler 1 is arranged on a casting platform, and has adiameter of 300 mm. The cooler, the hot top and the mold are concentric,and the bottom end of the uniform cooler 1 is flush with that of themold. The heat insulation end 15 of the uniform cooler 1 is made of hightemperature-resistant heat insulation ceramic material, and has adiameter of 300 mm and a thickness of 10 mm; the lower cooling end 17has a diameter of 350 mm, and is made of graphite; the blade is 3 innumber, and has a width of 50 mm. The uniform cooler 1 has a rotationalspeed of 60 r/min.

The electromagnetic coil is arranged on the periphery of the oil-gaslubrication mold 4, which may generate a rotating electromagnetic fieldthat applies shearing to an alloy melt, and the magnet yoke has a lengthof 50 mm.

During the semi-continuous casting operation, a melt that has beenrefined and stabilized to be 100 degrees Celsius higher than theliquidus temperature is poured into the hot top. Air and lubricating oilare introduced into the gas groove 12 and the oil groove 11 arranged onthe outer wall of the graphite ring 7, wherein the flow of air is 1430mL/min, while oil is supplied in a pulsed manner, with the oil supplyingcapacity being 80/s; the flow of the upper row of water spraying holes13 is controlled to be 20 L/min, while that of the lower row of waterspraying holes 14 is controlled to be 30 L/min; the speed of casting is65 mm/min. The alloy melt reaches the mold through the hot top 2, andthe liquid surface of the melt is elevated to a desired height; afterthe continuous casting operation is initiated, the dummy ingot 10descends slowly, and the flow of cooling water is increased slowly;after the casting process is stabilized, uniform cooling andelectromagnetic stirring are applied until the casting process iscompleted, wherein the cooling intensity of the uniform cooling is 1210W/(m²·k), and the shearing rate of the electromagnetic stirring is 110s⁻¹.

Through comparison of surface quality and internal structures betweenthe 7075 aluminium alloy rounded ingot (Φ=582 mm) prepared by the commonsemi-continuous casting method and that prepared by the presentinvention, it is found that the ingot prepared by the commonsemi-continuous casting method is featured by poor surface quality andcourse internal structures, as shown in FIGS. 5a and 6a ; however, theingot prepared by the present invention has a smooth surface, a fine anduniform internal structure, and an average grain size of 154 μm, asshown in FIGS. 5b and 6 b.

1. A device for preparing a large-sized high-quality aluminium alloyingot, comprising: a uniform cooler; a hot top; an oil-gas lubricationmold; an induction coil; and a dummy ingot, wherein the hot top isarranged above the oil-gas lubrication mold, the induction coil isarranged outside the oil-gas lubrication mold, the uniform cooler isarranged inside the oil-gas lubrication mold, and the dummy ingot isarranged below the oil-gas lubrication mold.
 2. The device for preparinga large-sized high-quality aluminium alloy ingot according to claim 1,wherein the oil-gas lubrication mold comprises a mold body and agraphite ring mounted above the mold body, wherein the graphite ring isprovided with a gas groove and an oil groove on its outer wall, andwherein the oil groove is separated from the gas groove, and is arrangedon an upper portion thereof.
 3. The device for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 2, wherein the gasgroove is divided into 3-20 sections for independent gas supply andcontrol.
 4. The device for preparing a large-sized high-qualityaluminium alloy ingot according to claim 3, wherein each section of thegas groove has a length of 100-500 mm.
 5. The device for preparing alarge-sized high-quality aluminium alloy ingot according to claim 2,wherein the graphite ring is made of porous graphite.
 6. The device forpreparing a large-sized high-quality aluminium alloy ingot according toclaim 2, wherein the mold body is provided with upper and lower rows ofwater spraying holes, and wherein the upper row of water spraying holesforms an angle of 15-30 degrees with respect to a wall of the mold, andhas a diameter of 1-5 mm; while the lower row of water spraying holesforms an angle of 0-25 degrees with respect to the wall of the mold, andhas a diameter of 2-8 mm.
 7. The device for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 6, wherein watervolumes of the upper and lower rows of water spraying holes areindependently controlled, and the diameter of the upper row of waterspraying holes is smaller than or equal to that of the lower row ofwater spraying holes.
 8. The device for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 1, wherein amagnet yoke of the induction coil is of a telescopic design, and isretractable in a range of 0-100 mm, and wherein the induction coilgenerates an electromagnetic field which is guided into a melt insidethe mold via the magnet yoke.
 9. The device for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 8, wherein theinduction coil generates a rotating electromagnetic field, a travelingwave electromagnetic field or a compound electromagnetic field.
 10. Thedevice for preparing a large-sized high-quality aluminium alloy ingotaccording to claim 1, wherein an upper portion of the uniform cooler isa heat insulation end, and a lower portion is a cooling end, wherein theheat insulation end is provided with a stirring blade.
 11. The devicefor preparing a large-sized high-quality aluminium alloy ingot accordingto claim 10, wherein the heat insulation end is of a cylindrical shape,and is made of high temperature-resistant heat insulation ceramicmaterial.
 12. The device for preparing a large-sized high-qualityaluminium alloy ingot according to claim 11, wherein the heat insulationend has an outer diameter of 100-800 mm.
 13. The device for preparing alarge-sized high-quality aluminium alloy ingot according to claim 10,wherein the cooling end is made of thermally conductive material. 14.The device for preparing a large-sized high-quality aluminium alloyingot according to claim 13, wherein the cooling end is of a spiralshape, and is made of graphite, copper, molybdenum, titanium orcomposite materials thereof.
 15. The device for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 10, wherein thestirring blade is made of high temperature-resistant material, and isarranged to be 0-8 in number.
 16. The device for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 15, wherein thestirring blade is made of copper, molybdenum, titanium, ceramic orcomposite materials thereof, and has a width of 10-100 mm and athickness of 2-8 mm.
 17. The device for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 1, wherein theuniform cooler is one or more in number, which is arranged to a heightwhere the mold is located, and has a rotational speed of 0-300 r/min.18. The device for preparing a large-sized high-quality aluminium alloyingot according to claim 17, wherein a cooling medium employed by theuniform cooler is air, nitrogen, water or oil, and the flow of thecooling medium is 0-2000 L/min.
 19. A method for preparing a large-sizedhigh-quality aluminium alloy ingot, the method comprising the steps of:pouring a melt that has been refined and stabilized to be 80-100 degreesCelsius higher than the liquidus temperature into a hot top during thesemi-continuous casting operation; introducing air and lubricating oilinto a gas groove and an oil groove arranged on an outer wall of agraphite ring; controlling flows of upper and lower water sprayingholes; an alloy melt reaches an upper portion of a dummy ingot throughthe hot top and a mold, and a liquid surface of the melt is elevated;after the continuous casting operation is initiated, the dummy ingotdescends slowly, and increasing a flow of cooling water slowly; andafter the casting process is stabilized, applying uniform cooling andelectromagnetic applied to obtain a large-sized high-quality aluminiumalloy ingot in the end.
 20. The method for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 19, wherein theflow of air in the gas groove is 500-5000 mL/min, while the oil groovesupplies oil in a pulsed manner, and has an oil supplying capacity of60-100/s.
 21. The method for preparing a large-sized high-qualityaluminium alloy ingot according to claim 19, wherein the flow of theupper row of water spraying holes is 1-50 L/min, while that of the lowerrow of water spraying holes is 20-100 L/min.
 22. The method forpreparing a large-sized high-quality aluminium alloy ingot according toclaim 19, wherein the speed of casting is 20-100 mm/min during thecontinuous casting operation.
 23. The method for preparing a large-sizedhigh-quality aluminium alloy ingot according to claim 19, wherein thecooling intensity of the uniform cooling is 500-5000 W/(m²·k).
 24. Themethod for preparing a large-sized high-quality aluminium alloy ingotaccording to claim 19, wherein the shearing rate of the electromagneticstirring is 10-2000 s⁻¹.